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I use Pythonwin as my Python IDE. In that environment, and I am sure in others, you can simply toggle it off. For Pythonwin, try the menu option View | Editor, then toggle off autocomplete.

EDIT I should add you are under no obligation to use the one provided in ArcMap, simply fire up your favourite IDE and >>> import arcpy which will give you full access to the arcpy module. If you have frequently used expressions, particularly complex ones associated with the spatial analyst module, use a text editor to save them, then cut/paste and modify to suit.

ESRI dev types are also touting pyscripter as a pretty full-fledge, open source Python interpreter...I would keep an eye open to see if this gets bundled at some stage as a replacement for Pythonwin. It has more capabilities and it rivals (apparently) some commercial Python IDEs
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